1. Field of the Invention
This invention relates to a striker for an automotive door latch apparatus.
2. Description of the Prior Art
A conventional striker for use in a door latch apparatus, as illustrated in FIG. 1, has a metallic base A securely bolted to a vehicle body and a roughly U-shaped metallic member B attached to the base A. When making the striker, a linear rod made of a high-strength steel having a strength equal to or higher than 70 kg/mm.sup.2 is cut to a part or fraction of an appropriate length, which is then bent roughly like a letter "U". Subsequently, the U-shaped member B is fitted into a jig D in such a manner that leg portions C and C thereof protrude from the jig D as illustrated in FIG. 2. Then, the leg portions C and C are inserted through a pair of holes E and E bored in the base A made of a normal-strength steel plate having a strength of 30 to 40 kg/mm.sup.2. Next, tip parts F and F of the leg portions C and C are pressed by members G and G, respectively, by simultaneously heating both of the leg portions C and C, with the result that an inner flange H and an outer flange I, between which the base A is firmly sandwiched as shown in FIG. 3, are formed in each of the leg portions C and C. Thereafter, the entire striker is heat-treated (namely, hardened and tempered). Thus the striker is finished.
There are four primary reasons for heat-treating the striker. First, is to increase the strength and toughness of the rod B. To attain this object, the hardening and the tempering are necessary. Second, is to arrange the structure of the rod B, which has become rough or coarse by heating the leg portions C, C while forming the flanges H, I. To achieve this object, the tempering is needed. Third, is to restore the toughness of the rod, which has been lowered by heating the leg portions C, C when forming the flanges. To attain this object, the tempering becomes necessary. Fourth, is to eliminate internal stress produced when bending the rod to shape the rod like a letter "U". To achieve this object, the hardening and the tempering become necessary.
Conventional heat treatment is performed after the base A and the U-shaped member B are put together, so as to attain the four objects in one cycle of the hardening and tempering. Thus, although the base A does not need to be heat-treated, the base A is heat-treated together with the member B. Consequently, the heat-treating cost increases largely. Moreover, when heat-treating both of the base A and the U-shaped member B simultaneously, the connection or binding between the base A and the U-shaped member B sometimes becomes loose owing to the difference in thermal expansion coefficient therebetween.
Further, to prevent the connection or binding between the base A and the U-shaped member B from becoming loose, there has been previously proposed a method in which the process of the heat treatment is divided into a plurality of steps or stages. In the case of this method, the hardening and the tempering are performed on the rod B alone after the rod is bent like a letter "U". Subsequently, the rod is connected with the base A. Thereafter, the tempering of the base is performed. This conventional method, however, has problems that the tempering of the base A is also necessary and that the number of steps composing the process of the heat treatment is increased.
Moreover, a striker, especially, a striker for use in a bonnet is sometimes painted together with a vehicle body after the striker is attached to the vehicle body. It is desired that the base A of such a striker is plated with zinc which excels as a rust-proof undercoat. A low-cost method for obtaining a zinc-plated base is to first purchase a galvanized steel and next perform a press working on the galvanized steel to form a base. Heat applied to the base A at the time of hardening or forming the flanges, however, melts and evaporates zinc. Therefore, the zinc-plating coating which is considerably expensive is performed on the striker after the heat treatment.